#1 Errors in math occur but are almost always caught by the bank if they're "unbalanced".
#2 You lose unless you can show it was the banks error
#3 Yes, mistakes happen
- do you use personal finance software like Quicken?
- when was the last time you knew you were in balance with the bank?
- look at the MICR encoding in the bottom right corner of your checks, that's the amount they actually posted for. Same with deposits.
Banks balance everything daily, all cash taken in and paid out to the cash on hand, deposited items to deposit slips. Mistakes happen and the exact amount of the error would give a good hint where to look. If it's divisible by 9 then it's likely a transposition (eg $527 instead of $257 would create an error of $270) in your case a $3 check posting for $300 would be a logical place to look and an easy error to make by adding a decimal. Also, if something posted to your account for an extra $300 then some other account
